An apparatus that displays a color image, such as a printer or a projection television, requires, as a light source, light sources of three colors: red (R), green (G), and blue (B). In recent years, as these light sources, there has been developed a wavelength conversion laser device (a laser system) that sets laser beams in the 900 nm band, the 1 μm band, and the 1.3 μm band as fundamental laser beams, and converts a fundamental laser beam into a second harmonic (SHG: Second Harmonic Generation) with a nonlinear material. In the SHG, to achieve a high conversion efficiency of the conversion from the fundamental laser beam to the second harmonic laser beam, the wavelength conversion laser device is required to increase a power density of the fundamental laser beam on the nonlinear material and to make the fundamental laser beam into a high-brightness laser beam with a small wavefront aberration.
A two-dimensional waveguide laser can increase a power density of a fundamental laser beam, so that the two-dimensional waveguide laser can achieve a high conversion efficiency of conversion from a fundamental laser beam to a second harmonic laser beam. However, the two-dimensional waveguide laser has a breaking limit due to the high power density, so that the two-dimensional waveguide laser is limited in high power operation. Furthermore, an output of an LD (Laser Diode) beam with a high beam quality in a two-dimensional direction connectable to a two-dimensional waveguide (in the same plane as the two-dimensional waveguide) is generally low, so that the two-dimensional waveguide laser is limited in high power operation.
Consequently, for the high power operation of the second harmonic laser beam, a planar waveguide laser in which a one-dimensional waveguide is structured is sometimes used. In the planar waveguide laser, to achieve high-power operation, a laser beam is oscillated in a spatial mode, i.e., in a direction perpendicular to a laser beam axis (a direction perpendicular to a principal surface of a flat plate) within a flat-plate plane thereby widening a beam diameter of the laser beam in the direction perpendicular to the laser beam axis or converting the laser beam into multiple beams. In such a planar waveguide laser in which a one-dimensional waveguide is structured, LD beams as a pumping source are just to be coupled in a one-dimensional direction in a planar waveguide. Thus, a high-power broad-area LD can be used in the planar waveguide laser in which the one-dimensional waveguide is structured, and as a result, a high-power laser beam can be obtained. Furthermore, a multi-emitter LD in which light-emitting points of LD beams are arranged in the one-dimensional direction can be used in the planar waveguide laser in which the one-dimensional waveguide is structured, and thus it is possible to obtain a higher-power laser output than the broad-area LD is used (see Non-patent document 1).    Non-patent document 1: IEEE J. Quantum Electronics Vol. 39 (2003), 495